1. Field of the Invention
The present invention relates to a method of determining a glucose concentration in a target by using near-infrared spectroscopy, and particularly a method of non-invasive determination of a glucose concentration in the blood of a subject, which can be used to a health examination at home, or a blood sugar measurement for subjects such as a diabetic at medical facilities.
2. Disclosure of the Prior Art
Near-infrared spectroscopy has been widely used in various technical fields such as agriculture, food industry, or petrochemistry because it is a kind of non-destructive inspection, and does not need a peculiar operation for preparing a sample to be inspected. Since near-infrared radiation is a low energy electromagnetic wave, it is possible to avoid the occurrence of radiation damage of the sample. Near-infrared radiation is difficult to be absorbed by water as compared with intermediate-infrared radiation, therefore, it is possible to inspect a sample in an aqueous solution state. In addition, there is an advantage of a high transmittance of near-infrared radiation into a living body.
On the contrary, an intensity of absorption spectrum within a wavelength range of near-infrared radiation is very weak, e.g., about 1/100 of the intensity of absorption spectrum within a wavelength range of intermediate-infrared radiation. In addition, there is a problem that it is difficult to clarify the assignment of an absorption spectrum detected from the living body by the use of near-infrared radiation. These problems prevent an accurate quantum analysis of the glucose concentration by using the near-infrared spectroscopy.
U.S. Pat. No. 4,655,225 discloses a spectrophotometric method for non-invasive determination of glucose concentration in body tissues. A light provided from a directional optical light source is irradiated on a selected body portion, and then a resulting radiation emitted from the body portion is collected. The collected radiation includes at least one band with a wavelength of 1575 nm, 1756 nm, 2100 nm, and 2270.+-.15 nm, typical of the glucose absorption spectrum, and at least one band with a reference wavelength in the range of 1000 nm to 2700 nm, typical of the absorption spectrum of background tissue. The absorption of glucose is nil or insignificant at the reference wavelength. After the collected radiation is converted into electrical signals, the glucose concentration of the subject is calculated by an electronic computer according to the electrical signals.
On the other hand, U.S. Pat. No. 5,070,874 discloses a method of non-invasive determination of the concentration of glucose in a patient. A near-infrared radiation over a limited range of wavelengths about 1660 nm is projected on a portion of the patient's body, and then the resulting radiation emitted from the portion is sensed. An expression for the magnitude of the resulting radiation as a function of wavelength is derived. The second derivative of the expression in a very narrow range at about 1660 nm, e.g., between 1640 nm and 1670 nm, is expanded. The glucose concentration of the patient is determined from the intensity of the resulting radiation at the maximum or minimum point of this derivative.
By the way, when determining the glucose concentration in a living tissue by using near-infrared spectroscopy, there is a tendency that absorption spectrums of water and components in the living tissue except for glucose overlap the absorption spectrums of glucose. FIGS. 12 and 13 show absorption spectrums of water, glucose (powder), albumin (powder), and cholesterol (powder), which are detected over wavelength ranges of first and second harmonic tones, respectively. For example, when an absorption spectrum of a target including water, glucose and albumin, is detected, it is expected that the absorption spectrums of water and albumin overlap a broad beak of the absorption spectrum of glucose at the vicinity of about 1580 nm, as understood from FIG. 12. In order to improve the accuracy of quantitative analysis of the glucose concentration, it is important to consider the influence of disturbance factors into the absorption spectrums of glucose.
Thus, there is room for further improvement in the methods of determining the glucose concentration of the prior art.